The high intensity discharge lamp has conventionally been used for outdoor or factory illumination. Among desirable features of these lamps are a very favorable spectral color distribution, high efficiency, much smaller size and the least property changes with changes in its lighting angle. Such lamps may be used for illuminating the inside of structures such as stores. An improvement over these features is provided by the use of a single ended metal vapor discharge lamp as a light source. For example, U.S. Pat. No. 4,415,829 describes a miniature direct current lamp and a structure of electrodes to maintain the desired cathode temperature. U.S. Pat. No. 4,308,483 teaches a lamp having a pair of electrodes whose terminal ends define an arc region having a given distance therebetween. This lamp is smaller in size and of increased brightness. U.S. Pat. No. 4,612,000 teaches a process for manufacturing a single-ended metal vapor discharge lamp, and discloses an isothermal operational condition with a fused silica envelope forming the discharge lamp. All of these improvements prevent particular problems, but problems still exist with these lamps. When these lamps are operated, the temperature of the coldest portion in the luminous bulb is likely to decrease greatly and prevent sufficient vaporization of the charge thereof when the direction of the lamp arc is changed, particularly by 90.degree. from a vertical to a horizontal position. This causes lamp properties, such as luminous efficiency and spectral color distribution, to be reduced by more than 20% in relative value.